A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In lithography systems, contamination of gas with carbon hydroxyl particles originating from substrate structures might damage optical elements, such as mirrors. Particularly, extreme ultraviolet (EUV) lithography systems might suffer from such gas contamination. To help prevent damage, a lithography system may be provided with a gas analyzing system to detect gas contamination.
To measure the concentration of a contaminant in a gas mixture, a residual gas analyzer (RGA) is often used as the gas analyzer. At a pressure exceeding a working pressure of the RGA, the measurement may be performed by diluting the gas mixture, e.g. by flowing it through an orifice of an inlet configuration into a reduced pressure chamber, and by connecting the reduced pressure chamber via a channel section of an outlet configuration to a pump system. By employing a reliable calibration, a concentration down to 10 parts per billion (PPB) may be measured.
For contaminant measuring, the main gas component of the mixture, also called the carrier gas, is often of no interest. The partial pressure of the main gas component, however, substantially determines the operating pressure in the reduced pressure chamber and at the RGA. Further, the dynamic range of the RGA and/or the lowest detection level of the RGA determines the lowest level of contaminant that can be measured.